The present invention relates to a process for purifying or working-up mixtures containing water, water-soluble salts, glycolate ions and organic solvents. The invention also relates to the use of such a process for working-up the liquid reaction media and washing media produced in the manufacture of polysaccharide ethers containing sodium-carboxymethyl groups.
In large-scale industrial processes, glycolic acid or its salts are frequently produced from monochloroacetic acid by alkaline hydrolysis. A reaction which is in principle comparable with the above also takes place as a side-reaction, when monochloroacetate ions, which result from monochloroacetic acid or its salts, are employed as an etherification agent for organic compounds containing hydroxyl groups, for example polysaccharides such as cellulose or starch. That is to say some monochloroacetate ions are also hydrolyzed to glycolate ions (hydroxyacetate ions), in addition to the reaction of the monochloroacetate ions with the hydroxyl groups of the organic compounds to form ethers. In view of the increasing importance of regulations mandating the least possible pollution of the environment by waste water, it is necessary to optimally purify waste waters before they are discharged into water systems. This is particularly true of waste waters containing organic by-products from chemical reactions which have a high chemical oxygen demand. At the same time, distillation of organic solvents, which are frequently employed in chemical reactions, for example as diluents or dispersant additives, consumes relatively large amounts of energy. Therefore, a very intensive search is being made for separation and purification processes for processing or working-up by-products of reactions, liquid reaction media and/or liquid washing media including for example, mixtures which particularly contain salts, water, organic solvents, acids and/or bases. However, the separation of relatively small residual amounts of salt, which are formed as by-products in chemical reactions and/or neutralization steps, from organic compounds is also frequently difficult, so that less expensive separation methods are also sought for such cases.
From the prior art for the production of carboxyalkyl ethers of polysaccharides, particularly cellulose, the following processes in which organic solvents are added may be mentioned as examples:
U.S. Pat. No. 2,517,577 describes a process for the preparation of water-soluble sodium carboxyalkylcellulose (NaCAlkC) in which an organic solvent is added to a suspension of cellulose, an aqueous alkali metal hydroxide solution and an etherification agent. n-Propanol, isopropanol, various butanols, acetone and dioxane are listed as suitable organic solvents. No information is given concerning the working-up of the organic solvents or the by-products of the reaction. Even the more recent U.S. Pat. No. 3,284,441 and U.S. Pat. No. 3,069,409, which describe modifications of the aforementioned process, contain no information on how to work up these components.
German Pat. No. 1,239,284 discloses a process for producing carboxymethyl starch (CMS) or carboxymethylamylose which is readily gelled in water. In this process the etherification reaction is carried out in a suspension which contains methanol, ethanol, acetone or isopropyl alcohol. The working-up of the intermediately produced sodium carboxymethyl starch (NaCMS) by suspending it in an aqueous medium containing hydrochloric acid and one of the listed organic solvents is also disclosed. However, no further information is given on the working-up of the organic solvents or of the by-products of the reaction.
The process for preparation of alkali metal salts of CMC (carboxymethylcellulose) according to U.S. Pat. No. 3,361,740 is carried out in the alkalization and etherification stages in such a manner that the reaction suspension contains an aliphatic alcohol having 2 to 4 carbon atoms and an aromatic hydrocarbon. No information is given concerning the working-up of the organic solvents or of the by-products of the reactions.
In the process for producing alkali metal CMC according to U.S. Pat. No. 3,347,855, a water/acetone mixture is employed as the liquid medium during the alkalization and the etherification of the cellulose. After the end of the reaction, the water/acetone mixture is separated from solid reaction products in a separator and is transferred to an acetone recovery column. In this column, excess water is removed from the water/acetone mixture. The acetone is returned to the reaction cycle. No information is to be found concerning the working-up of the by-products of the reaction.
Also in German Offenlegungsschrift No. 1,801,553, which describes the use of an ethanol/isopropanol mixture in a process for the preparation of alkali metal CMC, no information is given on working-up either of the solvent mixture or of the by-products of the reaction.
U.S. Pat. No. 4,017,671 and German Offenlegungsschrift No. 2,557,576 disclose, respectively, the continuous and non-continuous preparation of NaCMC in a reaction suspension containing isopropanol. After carrying out the reaction, it is possible to separate the aqueous isopropanol from the solid reaction components by mechanical means, and to return the aqueous propanol to the reaction mixture without additional treatment such as distillation. No information is to be found concerning the working-up of the by-products of the reaction.
Although Ullmans Encyklopadie der technischen Chemie (Encyclopedia of Technical Chemistry), Verlag Chemie-Weinheim, 4th edition-1975, volume 9, pages 192 to 212 states, under the heading "Celluloseather" (cellulose ethers) on page 201, right-hand column, that etherification agents are removed from the actual reaction by being hydrolyzed, and this therefore represents a substantial burden in the economics of cellulose etherification, and on pages 203/204 that essentially NaCl and Na glycolate--depending on the method of carrying out the reaction--are produced as by-products of the etherification of cellulose with monochloroacetic acid or its salts to give NaCMC or CMC, no information is given concerning the working-up of these by-products. In describing a procedure for purification of crude NaCMC, it is stated that the organic solvents used for this purpose can be purified by continuous distillation and re-used.
The following are examples of publications relating to the working-up of reaction mixtures containing glycolic acid (hydroxyacetic acid), its salts or similar compounds, and by-products formed in the reaction:
German Offenlegungsschrift No. 2,305,170 describes a three-stage process for the preparation of aqueous, highly concentrated glycolic acid nitrile solutions. In the first two stages of this process, hydrocyanic acid is added onto formalin in a thermostatically controlled column or vessel, and in the third stage the reaction products are after-treated in a cation exchanger. Alkali metal and alkaline earth metal cations formed during the reaction are removed from the reaction mixture in the strongly acidic ion exchanger, and the aqueous solution leaving the ion exchanger has a pH value of .ltoreq.2.
U.S. Pat. No. 4,054,601 discloses a process for purifying aqueous crude glycolic acid solutions produced by acidic and thermal hydrolysis of glycolic acid nitrile. The crude glycolic acid is removed from the salt-containing reaction medium by counter-current extraction with an organic solvent mixture of a neutral phosphoric acid ester such as tributyl phosphate and an aliphatic ether such as diisopropyl ether, and the glycolic acid is recovered again from the solvent mixture in the form of an aqueous solution by counter-current extraction with water.
In the preparation of glycolic acid from the mother liquors in the production of monochloroacetic acid according to German Offenlegungsschrift No. 2,810,906, the reaction mixture of glycolic acid, dichloroacetic acid, water and alkali metal chloride formed during the thermal hydrolysis of the optionally neutralized mother liquor is extracted with a preferably water-insoluble organic solvent such as a halogenated hydrocarbon. In this extraction stage, the dichloroacetic acid is substantially removed from the reaction mixture. Glycolic acid and alkali metal chloride primarily remain in the aqueous phase, whereby a major portion of the salt can be precipitated in crystalline form and separated by concentrating the solution, for example, by carefully distilling off the water in vacuo.
According to German Offenlegungsschrift No. 2,810,975, the inevitable formation of salts during the alkaline hydrolysis of monochloroacetic acid is avoided by carrying out the hydrolysis thermally in the absence of alkali metal hydroxides.
In German Offenlegungsschrift No. 2,812,682 and German Offenlegungsschrift No. 2,812,683, sodium chloride is removed from aqueous glycolic acid solutions produced by alkaline hydrolysis of monochloroacetic acid, by adding to the solutions a water-miscible organic solvent, such as acetone or 1,4-dioxane, or an organic solvent which is water-immiscible or has only a limited miscibility with water, such as diethyl ketone, methyl isobutyl ketone or dibutyl ether. Depending on the type of organic solvent, either the solvent or the water is then removed from the mixture by distillation, and sodium chloride is precipitated from the mixture either when the organic solvent is added or only when the water is distilled off. In addition to monomeric glycolic acid, oligomeric and polymeric forms (glycolides, polyglycolides) which represent a lactide or polyester form are also obtained in most of the previously described processes.
However, the processes known from the prior art have the following disadvantages:
Although the complete distillative work-up of liquids containing water and organic solvents, such as liquid reaction media or washing media, can result in recovery of the organic solvents employed, it consumes large amounts of energy and is therefore a significant cost factor. PA0 The distillation residue or "bottoms" remaining after distillative work-up of such liquids contains--depending on the composition of the liquid mixture to be distilled--a large number of compounds, particularly salts and low volatility organic solids, the latter resulting in a high chemical oxygen demand (COD) of the waste water. PA0 Extraction of such distillation residues with the most diverse organic solvents in order to separate organic by-products such as sodium glycolate or glycolic acid from the usual salts such as sodium chloride leads in turn to the problems of working-up large amounts of organic solvent by distillation.